The present invention relates generally to the microwave ferrite devices and more particularly to an improvement of heat transmission from a surface mount termination that is the main heat source in isolators, to the mounting base, which, in operation, is installed on a heat sink. It specifically relates to isolators having a resistive termination to shunt the reflected energy to the ground in combination with a ferromagnetic housing for closing a magnetic loop in magnetic chamber.
There are contradictory requirements to the isolator housing in conducting the magnetic flux, and, at the same time, increasing the heat transmission. For a magnetic flux the housing made of mild steel is needed—the material having high magnetic susceptibility (μ>1000) but low coefficient of heat transmission (0.00062 BTU per second). For a good heat transmission the materials like copper or aluminum are needed (0.00404 and 0.00203 BTU per second, respectively) which do not have any magnetic susceptibility (they are diamagnetic). Therefore, many attempts have been done in the past to improve isolators' performance by combining these two materials in the most effective way. For example, Naohiko Kanbayashi teaches (U.S. Pat. No. 3,621,476) a nonreciprocal device in which some portions of a heat dissipating plate or heat sink are introduced into a magnetic chamber through apertures thereof and are made in close contact with the chamber (which houses microwave ferrite elements and a center conductor). This structure, however, is pretty complex and does not cover the isolators wherein the resistive element, a termination—the most substantial source of the heat, is situated outside of the magnetic chamber.
Also known is a prior art where one portion of the housing having a steel magnetic chamber with ferrite elements and a center conductor, and the other portion made of copper or aluminum where the termination is located. This prior art is shown in FIG. 1A wherein the termination 1a is situated in the copper/aluminum portion 2a (shaded by dots) and secured to the steel portion 3a by screws 4a. Ferrite elements (not shown) and the center conductor 5a (lead portions are only seen) are situated in the steel magnetic chamber which is closed by a steel cover 6a that closes the loop of the magnetic flux within the portion 3a. Thus, both contradictory requirements are reconciled. The drawback of the structure is the complexity of the two-portion alignment (a close coplanarity tolerance of overall structure is needed to create a flat common surface of the mounting base) and, accordingly, relatively high labor amount and cost are involved in assembly process. This two-portion structure also has lower reliability in handling and operation as compare with one portion housing devices.
Prior art with one-portion housing isolator is also known (see FIG. 2A). In this structure an intermediate copper/aluminum plate 7a is used, which is situated between the housing 8a and the termination 9a. So, at least a part of the heat transmission path passes in the material with much higher heat transmission coefficient than that in the steel housing. There are also drawbacks in the design. Firstly, part of the dissipation path still remains in a steel housing causing in operation a possibility for termination to be overheated. Secondly, the plate 7a needs to be secured in the housing 8a without any voids. Otherwise, in case of moisture, say, condensation, a detrimental galvanic couple of dissimilar metals can be formed in the voids, causing corrosion.
Thus, what is needed is an isolator that can provide both good magnetic susceptibility in the magnetic chamber and high coefficient of the heat transmission in the heat path from the termination to the mounting base of a device. This isolator should be of simple structure, easy to assemble and reliable in operation.